Lecture #22: The Physiology & Biochemistry of Vision
I. EXAMPLES OF
MISCELLANEOUS RECEPTOR CELLS
1. Sensory
transduction by a taste receptor (chemoreception). (Fig. 49.2)
2. Sensory receptors
in human skin (mehanoreception & thermal reception). (Fig. 49.3)
3. Sensory
reception by a hair cell of the ear (mechanoreception) (Fig. 49.4)
4. Eye cup
in planaria have pigments allowing sense of radiant energy without image. (Fig. 49.7)
5. Compound
eyes of insects and crustaceans consist of several thousand light detectors
(ommatidia), each which function as individual eyes and result in a
system that is ultrasensitive to movement because of multiple focal
points. (Fig. 49.8b)
6. Single-lens
eye in jellyfish, polychaetes, spiders, mollusks, and vertebrates.
II. THE HUMAN
VISUAL SYSTEM: (Fig 49.9)
1. Brief review
of anatomy:
(a) functions of sclera
(collagen), chorioid (pigment), and retina (receptors).
(b) functions of
aqueous humor, lens, and vitreous humor (moisture and focusing).
2. Human
retina has 125 M rods and 6 M cones (= 70% of all sensory receptors). (Fig 49.11a)
(a) rods
are mainly in lateral parts of retina and important for night vision.
(b) cones
are concentrated in fovea (150,000/mm2).
3. Rod and
cone discs are imbedded in chorioid layer.
4. Outer segment
of these receptor cells consists of discs containing visual pigments.
5. Visual
pigments of rods: (Fig. 49.11b)
(a) retinal
is the pigment of rods (a derivative of Vitamin A).
(b) opsin
is a protein partner of retinal that determines absorbing ability. (serpentine
receptor
(c) retinal bound
to opsin is called rhodopsin. (Fig. 49.11b)
6. Visual
pigments of cones:
(a) retinal
is also the pigment in cones.
(b) opsins are
of 3 variations, depending on red, green, or blue cones.
(c) collectively,
the retinal plus opsins are called photopsins.
7. Light causes isomerization
of the cis form of retinal. (Fig. 49.12)
8. The
isomerization causes signal transduction that closes Na+
ion channels and hyperpolarization of rod (Fig. 49.13)
8. The
hyperpolarization causes rod to actually release less neurotransmitter onto
bipolar cells. (Fig. 49.14)
9. Bipolar
cells synapse with ganglion cells with axons of optic nerve. (Fig. 49.15)
10. Horizontal and amacrine cells enhance the contrast and sharpen the image that is transmitted through the lateral geniculate nuclei to the visual cortex. (Fig. 49.16)